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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> question about biasing current https://designers-guide.org/forum/YaBB.pl?num=1151393934 Message started by hunk on Jun 27th, 2006, 12:38am |
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Title: question about biasing current Post by hunk on Jun 27th, 2006, 12:38am hi dear all: i have been questioned about the biasing current: (1) Why do we use those biasing current which vary with temp? i find the biasing current of many many circuits come from the BGP circuit, so its amplitude must change with temperature. i think if we use the constant current the performance of circuit may be better than those using varying circuit.(just guess) (2) Can i use the un-changing current to bias every circuit? many thanks!! hope you can help me! |
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Title: Re: question about biasing current Post by raf on Jun 27th, 2006, 5:56am There are many uses. For example: Varying current tracking a varying resistor will give you a constant voltage over PVT. |
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Title: Re: question about biasing current Post by mikki33 on Jun 27th, 2006, 7:32am You may use PTAT current in amplifiers partially compensate for the band width reduction. And, yes, you may use constant current to bias particulary any circuit... |
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Title: Re: question about biasing current Post by hunk on Jun 27th, 2006, 5:17pm raf wrote on Jun 27th, 2006, 5:56am:
this is a good idea. thanks! by the way, i know PTAT current is used in bipolar circuit to hold the gm constant. but in cmos circuit(i.e., cmos amplifier), what advantage is using PTAT current? |
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Title: Re: question about biasing current Post by hunk on Jun 27th, 2006, 5:27pm mikki33 wrote on Jun 27th, 2006, 7:32am:
thanks! Mikki, like the above reply, i know PTAT current is used in bipolar circuit to hold the gm constant. but in cmos circuit(i.e., cmos amplifier), what advantage is using PTAT current? and could you explain the reason for band width reduction in detail. why do many CMOS circuits (i.e., amp, comp, OTA,...) still use PTAT current? i plan to use constant current in my circuit. but i am afraid there may be some reason for using PTAT i have not known. so i want to ask for your help. |
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Title: Re: question about biasing current Post by mikki33 on Jun 30th, 2006, 12:30am Band width is gm/Cload. gm is sqrt(2*μn*Cox*(W/L)*Id) With temperature μn is reduced, so increase of Id is compensating for gm reduction and gm/Cload |
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